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Cheap, ultra low power RTC to periodically wake microcontroller from power-down?

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jbb:
I have heard - and might be wrong! - that IO pin drivers need quite a lot of silicon area. So the extra I2C logic is probably quite cheap compared to 2 IO pins and a larger plastic package required by SPI.

I think that quite a few MCUs offer separate crystal oscillators and Real Time Clock logic so that the device can wake up from RTC.

We’ve done that with the deeper sleep modes in an STM32.  MSP430 devices also offer it (but I haven’t tried myself).

jhpadjustable:
I just checked that the tinyAVR 1-series, particularly ATtiny(2|4|8)14, does have an external 32k oscillator and typically, at <85°C, consumes about 1-1.5µA in standby mode with that and the RTC running. Active current can be as small as 8µA (but there are minimum speed requirements to access the RTC registers, if you're not just using the periodic interval timer to wake up). Still only 54¢/1 from Digi-Key, looks like 4mm-wide SO-14 only.

I wanted a rude username:
Thanks for the feedback, kind people.


--- Quote from: krish2487 on December 03, 2019, 12:37:31 pm ---https://www.digikey.com/product-detail/en/texas-instruments/TPL5110DDCT/296-38830-2-ND/5130143

--- End quote ---

If the period of waking the microcontroller is known in advance, for 30¢ this is a good solution. Also its brother the TPL5010, which wakes the microcontroller directly over an interrupt line.


--- Quote from: Yansi on December 03, 2019, 01:08:04 pm ---For example STM32L011, if you want something small with low consumption.

--- End quote ---

540 nA with RTC and full RAM retention for 70¢ is very nice. You're right, this looks like a better approach overall.


--- Quote from: Howardlong on December 03, 2019, 01:22:16 pm ---in many cases, the family feature sets are limited compared to more mature and industry standard product lines. If you need to spend $0.50 to add the right functionality to a $0.10 MCU, it quickly negates the cost benefit.

--- End quote ---

Agreed. The window for suitable applications is narrow. They are good for things like toys and appliances in which the user wakes the microcontroller through a button press or other hard interrupt. Step outside this range and the best recourse is a more richly featured microcontroller.

I was trying to see how far I could push that window open, but clearly not far enough to squeeze my far arse through.  ;D

texaspyro:
I once built some data loggers.   I used a cheap micro (can't say which one) that supported using a watch crystal.  That micro's only purpose was to periodically wake up the main  instrument.   One of these loggers was lost for over 25 years.   When it was recovered it was still ticking and logging.   The log memory was sized for 3 years of data and was expected to be retrieved and dumped every year or two... the log queue wrapped around over 8 times.  The wakeup clock had drifted less than a hour.   The power for the loggers was a lithium primary "F" sized cell.

Peabody:
If the processor doesn't sleep very soundly, or if there are other circuit components that have the same problem, then it seems you could use an RTC module or similarly efficient tiny processor, powered by its own coin cell, to periodically turn on the power to the main circuit instead of just waking it up.  Then the main processor would maintain the power on while it's doing its thing, then shut off the power.  The power switching circuit would just be a P-channel mosfet, an NPN and a couple resistors, and use one GPIO pin.  And then you wouldn't really have to worry about sleep current or a regulator's quiescent current or current used in dividers since the whole thing would only be powered up briefly.

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